| Loop heat pipe (LHP) is a type of efficient and passive heat transfer device utilizing phase change of the working fluid. Owing to its many advantages such as high heat transfer capability, low thermal resistance, long transfer distance, and lack of moving parts, LHP has broad application prospects in the fields of electronic cooling with high heat flux and thermal control for spacecraft.This thesis introduces the working principle and characteristics of LHP, and the heat transfer in the evaporator is analyzed. The influence of heat leak from back conduction and side wall conduction on the operation of LHP is elaborated. In order to reduce the influence of side wall heat leak on the system, the welding type evaporator is adopted and the performance is improved. When the evaporator wall temperature is lower than85℃, the maximum operating heat load increases from140W to240W.In order to reduce the influence of heat leak from side wall conduction and improve the startup performamce of LHP, a novel LHP with two primary wicks is presented. In the experiments, the evaporators with O-ring and welding structure are respectively adopted, and the operating characteristics with different filling ratios, heat sink temperatures, working fluids and elevations are tested. The mechanism of temperature fluctuations is analyzed. And the experimental results show that the novel LHP with two primary wicks can start up successfully at low heat load of10W. The filling ratio has a great influence on the transition of the operating mode in the LHP with two primary wicks. With gravity assist, the LHP may operate in the mode of gravity controlled or capillary controlled. The operating temperature variations within low heat load range in this LHP present different trend compared with previous work. The influence of the wick in the evaporator back on the performance is related to the applied heat load and the distribution in the bifacial evaporator.The LHP with two evanorators is studied for the heat dissinated system with multinle heat sources, and the operating characteristics of startup, variable heat load and heat sharing are tested. The system can start up successfully with heat load applied on the two evaporators, and the operation patterns can change from the temperature fluctuation into the steady operation at low heat loads. The system shows the advantage that the low heat load startup can be improved when high and low heat loads are applied to the two evaporators, respectively. Increasing the filling ratio, the system performance improves, and the flow distributions of the returning liquid for each evaporator can impact the heat sharing in the system.The biporous wick with high porosity and high permeability is developed and applied in the experimental system, and shows an excellent performance. The flow and heat transfer in porous media is simulated by using lattice Boltzmann method (LBM), and the flowing details in porous media are obtained. And the effective thermal conductivity of porous media with multiphase fluid is predicted. The phase change model of LBM based on free energy is used to simulate the growth and departure of bubbles in compensation chamber, which has certain guide for the design of the wick and is the fundament of the mesoscopic simulation of the evaporation in the wick.In this thesis, the sealing process of the evaporator is improved, and a novel LHP with two primary wicks and a LHP with two parallel evaporators are presented. The thesis studies the startup and operating characteristics of the flat-type LHP by theoretical analysis and experiments. The influences of the evaporator sealing process, filling ratio, heat sink temperature, working fluid and gravity title angle on the performance of the flat-type LHP are analyzed. The study results have a certain guiding significance for engineering application of the flat-type LHP. |
PDF Full Text Request